This invention is directed generally to improvements in television receivers. It is particularly directed to a horizontal countdown system for developing horizontal rate pulses from a color oscillator signal.
In recent years, attempts have been made to replace much of the analog circuits in television receivers with digital circuits. For example, the conventional analog circuits for developing vertical rate pulses have, in some receivers, been replaced by a digital vertical countdown system. In receivers using such a countdown system, the vertical oscillator is eliminated and the horizontal oscillator is modified to produce horizontal rate pulses whose frequency is twice the normal (15.73 kilohertz) horizontal frequency. These higher frequency horizontal rate pulses are commonly referred to as "2H" pulses, and they are divided or "counted down" by a constant factor to develop vertical rate pulses. To drive the receiver's horizontal scan at the nominal 1H rate, the 2 H pulses are merely divided by a factor of 2.
To further digitize television receivers, it has been proposed that a horizotnal countdown system be included to develop the 2H pulses. In this approach, the color oscillator is used as a reference, and its frequency is divided or counted down to develop the 2 H pulses. The latter pulses are then counted down by a conventional vertical countdown system to provide vertical rate drive.
The prior art has taken the following approach to implement a horizontal countdown system. The color oscillator is modified to operate at a frequency of 4Fc (four times the nominal color subcarrier frequency of 3.58 megahertz). A counter counts the 4Fc pulses developed by the color oscillator, and a decoder senses when the counter has reached a count of 455. When that occurs, the decoder develops an output pulse and resets the counter to begin counting anew. Each time the counter reaches a count of 455, the decoder develops another output pulse. These output pulses occur at a frequency of 2H and are divided by two to drive horizontal scan. They are also input to a vertical countdown system to drive vertical scan.
One problem with the approach described above occurs when the transmitted television signal does not include a color burst. Under this condition, the receiver's color oscillator becomes free running, and the decoder's output does not remain locked to 2 H. Consequently, vertical and horizontal synchronization is lost between the receiver's scan and the transmitted signal.
Another problem occurs when the signal applied to the receiver is developed by a non-standard signal source, such as a television video game, a television playback device or the like. Such devices do not always output a signal whose color frequency and horizontal frequency are locked to each other according to the NTSC standard. Hence the decoder's output may not be developed in synchronism with the horizontal sync pulses which are received from the non-standard signal source.
To overcome this problem, the prior art employs a detector to sense the lack of synchronism and to modify the operation of the decoder (or the counter) so that the decoder develops an output pulse on a count different from 455. Because the decoder (or the counter) cannot be easily modified to operate at a count which differs from 455 other than by an integer or a multiple of an integer, it is still difficult to achieve exact synchronism. For example, the incoming television signal may require that the decoder develop an output pulse at a count of 455.4. Under this condition, the decoder may develop an output pulse on a count of 455 during one line, and develop another output pulse on a count of 456 during the next line. This creates a phase error between the decoder's output and the incoming horizontal sync pulses. The magnitude of the phase error can be reduced by increasing the frequency of the color oscillator, but precise phase synchronization is still difficult to achieve.
For the foregoing reasons, digital horizontal countdown systems have not been used in television receivers. The present invention overcomes those problems in a manner which makes it commercially feasible to replace a receiver's analog horizontal system with a digital horizontal countdown circuit.